For many years, the standard call routing method used by most North American telephone compares has been fixed hierarchical routing (FHR). In networks employing FHR, calls are sent from their origin network node to their destination network node along routes comprising a predetermined sequence of connection paths using circuit groups (trunk groups) between successive nodes. At an origin node, a call is offered first to a direct circuit group. If all direct circuits am busy, the call is routed via a tandem node. Should this alternative route prove to be busy as well, the network continues to search for a path, selecting alternative routes in a fixed, predetermined sequence from the most-direct to the least-direct route. Finally, if no connecting route can be found, the network blocks the call. The predetermined sequence of alternative routes is programmed into the network equipment and is "fixed" in the sense that any changes require human intervention.
While FHR remains the most prevalent routing method in North America, a number of developments, for example new services, deregulation, have led suppliers of telephone services to seek a more flexible alternative. As a result, in recent years, networks employing dynamic traffic management have been introduced, particularly so-called Dynamically Controlled Routing (DCR) networks which make use of the capabilities of modern stored program control switches with high-availability, real-time data processing abilities and the "intelligence" of a computer called the network processor (NP) to monitor the network and change the routing patterns periodically, in "near real-time". A general discussion of FHR and DCR networks appears in an article entitled "Dynamically Controlled Routing" by Hugh Cameron and Serge Hurtubise, in Telesis, Vol 1, 1986, published by Bell-Northern Research. An early version of dynamically controlled routing is disclosed in U.S. Pat. No. 4,284,852 issued August 1981.
DCR yields significant advantages in network control. Thus, it ensures the efficiency of switching equipment by preventing excessive traffic from reaching the machine; inhibits the spread of switching congestion; maintains circuit-group/trunk efficiency by reducing the number of links per call when facilities are congested; and makes full use of available idle facilities. Of particular advantage is the DCR system's capability of coping with failure of a switch. The DCR network processor can download routing instructions for neighbouring switches enabling them to use available links to carry tandem traffic around the failed switch. These features of DCR greatly improve the percentage of calls which are successfully routed through the network. Even so, increasing competition amongst suppliers of telephone services has motivated them to find ways of using the network equipment more efficiently to reduce costs and improve the completion rate of traffic handled by their networks.
One aspect susceptible of improvement is the procedure governing the way in which a call exits the DCR network for a final destination outside the DCR network, possibly in another network. Known DCR networks use a procedure inherited from FHR known as Unique Exit Gateway. Since there are many different destinations, most of them outside the DCR network, each final destination is associated with an intermediate destination node within the DCR network. Calls in the DCR network which are destined for a particular final destination are then routed to the associated intermediate destination node. From the intermediate destination node, the calls are routed to the final destination, possibly using other networks than the DCR network. If the intermediate destination node is affected by congestion or a failure, all calls associated with that intermediate destination node will be affected. Although other networks elements might be able to route calls to the final destination, the DCR network is not allowed to use them as alternative exit points. This detracts from the improvements in survivability that can be achieved by DCR networks in case of equipment failures.